1. Field of the Invention
The present invention relates to a multi-path type ink-jet printing apparatus, and in particular to, an ink-jet printing apparatus suitable for printing data in full colors.
2. Prior Art
In an ink-jet printing apparatus, when one session of a print scanning operation of an ink-jet head is performed, all nozzles are set to be enabled. The print scanning operation is performed such that the ink-jet head is moved for a length corresponding to the number of nozzles for each print scanning operation in a direction perpendicular to the print scanning direction.
However, this method has the following problem. As shown in FIG. 13, assuming that the band width of which an ink-jet head 31 can print data on a sheet 32 with one print scanning operation is denoted by W, the head 31 prints data in bidirectional directions in such a manner that the head 31 is vertically moved by the band width W whenever each print scanning operation is performed as denoted by arrows (dotted lines). Thus, when output data 33 of the ink-jet head 31 exceeds two or more bands, each band deviates for .DELTA.d due to the feed deviation and feed error of the head 31. When the band width W is as large as several mm, since such a deviation periodically takes place, the picture quality deteriorates. However, it is known that when such a deviation takes place in a very short period (for example, around 1.5 mm), eyes of human beings cannot substantially recognize it.
Due to this fact, a multi-path method has been used.
In a multi-path type ink-jet printing apparatus, part of all nozzles of the head, which are interspersedly selected, are set to be enable at a time in one print scanning operation. In the multi-path method, data that is to be output in one print scanning operation is printed with a plurality of print scanning operations. At this point, whenever the print scanning operation is performed, the head is slightly moved in the vertical direction.
FIGS. 14 and 15 are schematic diagrams for explaining a four-path type printing method.
As shown in FIG. 14, assuming that the number of nozzles of the ink-jet head 31 is 16 and the first print scanning time is T, in a print scanning operation, nozzles N11, N21, N31, and N41 are enable. In the second print scanning operation, nozzles N12, N22, N32, and N42 are enable. In the third print scanning operation, nozzles N13, N23, N33, and N43 are enable. In the fourth print scanning operation, nozzles N14, N24, N34, and N44 are enable. Thus, in each print scanning operation, four nozzles disposed at intervals of four nozzles are successively selected. As shown in FIG. 15, when each print scanning operation is completed, the head 31 is vertically moved for W/4. After a total of four print scanning operations are completed, data for the band width W is printed. In each scanning operation, even if output dots deviate, the picture quality does not remarkably deteriorate.
Now, as shown in FIG. 16, when one straight line is printed, the time difference after a dot d1 is formed in the first print scanning operation until a dot d2 adjacent to the dot d1 is formed in the second print scanning operation is equivalent to a period T of one print scanning operation. Likewise, dots d3 and d4 formed in the third and fourth print scanning operations are formed the period T time after the dots d2 and d3 adjacent thereto are formed, respectively. However, a dot d5 formed in the fifth print scanning operation is adjacent to both the dot d4 and a dot d2' formed in the second print scanning operation. The time difference between the dot d4 and the dot d5 is T. However, the time difference between the dot d2' and the dot d5 is 3T. In this case, the ink dried state of the dot d2' is different from the ink dried state of the dot d4. Thus, the ink mixed state between the dots d5 and d4 is delicately different from the ink mixed state between the dots d5 and d2'.
FIG. 17 is a schematic diagram showing time differences of adjacent dots formed by the above-described head driving method in a longer range than that in FIG. 16.
As shown in FIG. 17, dots with time differences 3T against adjacent dots regularly take place in relatively large spatial intervals. These dots cause density differences, resulting in a picture deterioration as texture noise.